Speaker tracking on a single core in a packed based conferencing system

ABSTRACT

A distributed conferencing system having a plurality of conferencing nodes to connect groups of participants to a conference. Each of the conferencing nodes provides for the connection of one or more participants to the conference. Each node includes a DSP for distributed signal processing. The node DSP includes: A signal measuring device for measuring features of the signals from each of the participants, such as power, zero crossing rate and short term energy and voice activity determination and feature extraction from the signals of the participants connected to each node. Each node has a single core speaker tracking algorithm for determining the relative features of each of the number of participant input signals. The speaker tracking compares the characteristics of the speakers of the core and determines which speakers are to be included and which speakers are to be excluded from presentation to the other nodes in the conference. The core also has a communication device for communicating the measured signal characteristics for the included participant input signals to all other conferencing nodes and for receiving the signal characteristics from selected participants of the other nodes.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] None

FIELD OF THE INVENTION

[0002] The invention relates to voice tracking in telephony conferencing over packet networks. More specifically, the present invention relates to the tracking of speakers on a single core of a packet based conferencing system.

BACKGROUND OF THE INVENTION

[0003] Conferencing allows multiple users to participate in the same call on a dynamic basis. A single call can be established and additional users can be added or deleted on a dynamic basis.

[0004]FIG. 1 illustrates a representative prior art digital telephony conferencing system. The conferencing system consists of numerous participants at locations 1 through n. One or more telephones 5 may be located at each location. A multiplexing and demultiplexing device 4 connects each of the telephones to the conference bus 3. Some locations may have only a single user, while other locations may have multiple users. Each user will be individually connected through a multiplexer 4. A time-division multiplexing (TDM) bus 3 conveys the signal information generated by each participant to all other participants of the system.

[0005]FIG. 2 illustrates the use of a single core for processing the signals from the various locations illustrated in FIG. 1. All of the telephones 5 are connected to a common core, such as a DSP core 7. The DSP core 7 has a number of channels, each participant telephone 5 is accommodated by a channel. Each channel acts as a router for receiving the audio signals from the telephone connected to the channel. The core 7 also has a plurality of mixers 8 for providing audio signals out to each of the telephones 5. The mixer for each telephone sums the signals for all other telephones and provides this signal to the particular telephone served by that mixer. The mixer does not provide a telephone with its own signal because the speaker would hear an echo of his own voice.

[0006] Each of the signals from each of the telephones has noise associated with the signal. The noise can be background noise or other noise. The mixing operation, a summation of all signals, causes the noise level to increase at each of the telephones. The increase in noise can limit the number of participants and/or can have an adverse impact on the quality of the communication.

[0007] The use of a single core for a conference call also reduces the total number of participants which is limited by the number of channels on a single core. Further, the transmission of background noise, as well as the transmission of other speakers in addition to the primary speaker places an additional load on a communications system.

[0008] When a large number of people participate in a telephony conference, there is a significant likelihood that two or more participants will speak at the same time. It is also likely that background noise originating at one or more group nodes will be transmitted through the conferencing system. The transmitted signals of the background noise and multiple speakers interfere with the signal that is of interest to the conference participants. These interfering signals impair the ability of the conference listeners to hear and understand the speaker that is of interest to them and thereby reduce the quality of the communication.

SUMMARY OF THE INVENTION

[0009] The present invention reduces the unnecessary use of communication resources to support the communication of signals that will ultimately be muted and reduces the processing demands on a conferencing system.

[0010] The use of distributed processing and multiple cores by the present invention allows the number of conferencing nodes to be greater than the number of channels on a single core. All of the conferencing nodes may be set up at different cores. Alternatively, some of the cores of the distributed system may support more than one node. The bandwidth for communication between cores is less than the bandwidth within a single core. The limitation on communication bandwidth is addressed by the present invention. The present invention teaches active speaker tracking and mixing operation for improved performance in distributed multi-core conferencing systems.

[0011] The present invention utilizes low-delay voice activity detection. The present invention also teaches speaker tracking implemented for speakers from a single core and can be used with speaker tracking for speakers from multiple cores. The mixing operation is optimized based upon speaker tracking to reduce the cumulative noise summing effect and a communication algorithm which provides effective system control.

[0012] The system of the present invention is implemented with low complexity to minimize processor and memory resource requirements. The present invention provides high quality communications capable of tolerating a significant range of noise conditions.

[0013] A distributed intelligence conferencing system, as illustrated in FIG. 3, is disclosed, having a plurality of conferencing nodes 12 connected to a communication bus 11. Each of the conferencing nodes can connect one or more participants to the conference and has feature measuring devices for measuring various features of a signal, such as power level, zero crossing rate on each of a number of participant input signals; a processing device, interconnected with the feature measuring device, for determining the relative features of each of the number of participant input signals; and a communication device, interconnected with the processing device, for communicating the measured features and the signals for a plurality of participant input signals to the other conferencing nodes.

[0014] Each node 12 of the system measures the features of each of the conference participants connected to that node. The processor in the node identifies the significant participants based upon the measured features. The three significant signals are identified in the exemplary embodiment, however, a greater or lesser number may be identified depending on the implementation of the present invention. Each node 12 provides the signal features and signal sample of its three significant participant signals to the other nodes through the bus 11. The nodes then compare each of the three significant local signals to the features of the received signals and determines if the local signals should be included in the conferencing group signal provided to the participants in the conference.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Exemplary embodiments of the present invention are discussed with reference to the following drawings:

[0016]FIG. 1 is a block diagram illustrating a representative prior art digital telephony conferencing system.

[0017]FIG. 2 is a block diagram illustrating the use of a single core for mixing and processing signals from various locations.

[0018]FIG. 3 is a simplified block diagram illustrating a representative conferencing system of the present invention.

[0019]FIG. 4 is a block diagram of the functional configuration of an exemplary distributed conferencing system of the present invention.

[0020]FIG. 5 is a block diagram of the functional configuration of an exemplary distributed conferencing system of the present invention showing additional block detail.

[0021]FIG. 6 is a block diagram illustrating an exemplary representative structure of a conferencing node of the conferencing system of the present invention.

[0022]FIG. 7 is a state diagram illustrating the voice activity detection states of the present

[0023]FIGS. 8A through 8D are logic flow diagrams illustrating the voice activity detection of the present invention.

[0024]FIGS. 9A through 9D are logical flow diagrams illustrating speaker tracking for a single

DETAILED DESCRIPTION OF THE INVENTION

[0025]FIGS. 3, 4 and 5 illustrate a representative conferencing system of the present invention. FIG. 3 illustrates the overall physical structure. FIGS. 4 and 5 illustrate the overall functional or logical components of exemplary embodiments of the present invention. The conferencing system contains a number of conferencing nodes 12 identified by their respective group numbers group 1 through group n. Each node 12 is capable of providing conferencing support to multiple participants or users 14 that are connected to the node 12 by separate communication channels 15. Communication between the multiple nodes 12 is accomplished via a bus 11. The bus can be for example a TDM bus.

[0026]FIG. 4 also illustrates the voice activity detection 9, single core speaker tracking 10 and multiple core speaker tracking 13 within the DSP core. In the exemplary embodiment of FIG. 4, there is no local mixing of the speaker signals. The mixer 33 mixes the signals for each of the users 14 from the multi-core tracking 13 after communication from the TDM bus 11. FIG. 5 illustrates an embodiment with local mixing 34 as well as the global mixing 33

[0027]FIG. 6 illustrates an exemplary representative structure of a conferencing node 12 in greater detail. A number, n, of separate participants or users 14 are connected to this node 12 by communication channels 15. In an exemplary embodiment of the invention, node 12 is comprised of a DSP that is capable of supporting a number of communication channels 15. Each channel connects one of the users 1-n to the DSP. Each DSP may provide communication to one or more participants, depending on how many users are connected. In the exemplary embodiment, voice activity detection module 16 of the DSP, has voice activity detection 17 capabilities and receives the signals on the communication channel. The signal s also feed directly to the single core speaker tracking algorithm 23 and to the feature extraction module 18 which has feature extraction capabilities FTBx 19, to measure features such as the power, short term energy, long term energy, zero crossing level and other desired features of the signal on each communication channel 15 during some portion of the frame period, τ. In a preferred embodiment, the features are periodically obtained during each 2.5 msec of a frame period. The decision, ie voice or not voice v/n, from each VAD 17 is also sent to its respective FTGEx 19. Based upon the determination of the VAD, the FTEx will extract features from the signal if the signal has been identified as voice. The features fare sent to the single core speaker tracking algorithm 23 along with the signal s.

[0028] The extracted features f corresponding to the signals s, are provided to a single-core speaker tracking algorithm 23. The present invention recognizes that the feature extraction and signal from the individual participants may require signal processing, such as commonly applied to remove distortion or hystorisis. Single-core speaker tracking algorithm 23 compares all of the features of each of the users 15 provided to single-core algorithm 23 from module 18 that were measured during frame period τ and processes the information as described below in reference to FIGS. 7 and 8. Although the preferred embodiment of the invention identifies a certain number of signals for tracking, a different number of signals may be identified depending upon the specific implementation of the present invention. Once the single core speaker tracking algorithm has made its determinations, the output of the single core speaker tracking algorithm 23 is output 27 both to the multi-core speaker tracking algorithm 20 and to the multiplexer 28. Multiplexer 28 also receives the signals s and the extracted features f from the participants connected to the node. Multiplexer 28 mutes selected ones of the signals based upon the results of the speaker tracking algorithm. The muted signals are not placed on the conference bus 11, so that the conference listeners, including those in the current node, will not hear these signals. The non-muted signals and their associated features are placed on the conference bus 11 by the multiplexer 28.

[0029] Multi-core speaker tracking algorithm 20 receives the results 27 of the single core speaker tracking algorithm 23 receives the extracted features f of the signals passed by the Multiplexer 28 and also receives measured feature information 30 pertaining to other participants in other groups of conference participants that are connected to other conference nodes of the system. The feature information from participants at other nodes is provided to multi-core algorithm 20 through the bus lines 30 connecting it to a demultiplexer 29. The features of signals from other nodes is used to compare with the feature measurements of the signals from the current node to determine which signals from which participants connected to the conference will be transmitted to the participants 1 through n of the current node. A control signal CNTL is sent to the multiplexer 32 to instruct the multiplexer 32.

[0030] Demultiplexer 29 receives the TDM frame 40 from the TDM bus 11 and demultiplexes the feature information and selected signals provided by the other nodes. The feature level information is provided to multi-core algorithm 20 on data lines 30 and the selected signals are provided to multiplexer selector 32 on data lines 31. Multiplexer 32 provides the signals to an input of each of the switches 22 which in turn provide the signals to mixers 25 which in turn provide an input to an associated user. The switches 22 exclude the signal corresponding to the associated user, if such signal is included in the aggregate from the multiplexer 32. The mixers 25 provide the aggregate of all participant signals as an input to each associated participant's telephone, except for the input signal generated by the associated participant himself, to avoid an echo of one's own signal. A participant does not receive his or her own signal, just as one does not receive his or her own signal in the telephone speaker for any other type of telephone call.

[0031] Multiplexer 32 receives signals of participants from other nodes on lines 31 and signals of the current node participants s on line 24 from the multiplexer 28. Multiplexer also receives control signal CNTL from multicore speaker tracking algorithm 20. Based upon the control signal, multiplexer 32 send the selected signals to the participants of the current node through switches 22 and mixers 25. Because the signals sent from the multiplexer 32 may contain the signals of participants on the current node, switches 22 mute any signals which correspond to the user to which the switch is connected. Mixers 25 then mix the remaining signals and provide the signals to the users 1-n.

[0032] Each node separately pre-screens (single-core speaker tracking) the signals of the local participants, reducing the number of samples transported by the communications system. Because each node need only process up to three local signals and up to three signals from remote nodes, the total system processing bandwidth resources needed to support a conference call is distributed, can be conduced in parallel, and is reduced.

[0033] Speaker tracking consists of identification of the presence of speech on a signal from a particular participant and determining the relative strength or volume of that speaker compared to active speakers. Speaker tracking is used to identify the speakers of interest for transmission in the conference and for muting of other signals from other speakers.

[0034] In order to implement speaker tracking, voice activity detection must be properly implemented. In an exemplary embodiment, a low delay VAD with a configurable fixed energy threshold, as illustrated in FIGS. 7 and 8 can be utilized. The VAD utilizes a 2.5 ms block for power measurements and zero crossing rate for identification of the beginning and end of speech. Bach VAD frame can be divided into n blocks. The VAD makes decisions using 2.5 ms previous samples and current frame samples.

[0035] As illustrated in FIG. 7, the VAD has three states, Voice 41, Transition 43 and Noise 45. The state of the VAD is dependant upon the measured energy level Ei of the current signal block and the zero crossing rate and their distribution. A high threshold T_(high) and a low threshold T_(low), are used to differentiate voice and noise. Any signal energy blocks in the current frame above the high threshold T_(high) are classified as voice. Signal block energies in the current frame below the low threshold T_(low), are classified as noise. Signals between the two thresholds can be noise or voice and will be classified dependant upon the state of previous signal frame, background energies, zero crossing rates and their characteristics.

[0036] When in voice state 41, the VAD can remain in voice state or move to transition state. If the current Ei is above the high threshold T_(high) than the VAD will remain in voice state 41. If all the Ei's of the current frame are below the low threshold T_(low), than the VAD will move to transition state 43 from which it can move to noise state 45. The VAD will not move directly to noise state 45 to reduce the likelihood of missclassification of a weak voice signal as noise and to avoid frequent clipping which can be caused if the state machine moves to noise state during brief pauses in voice.

[0037] When in voice state 41, the state will remain voice state as indicated by arrow 42 when:

[0038] (a) any block energy (Ei)>high energy threshold (Th), or;

[0039] (b) voice counter>voice count threshold (Tvc) and current ZCRi>ZCR threshold, or;

[0040] (c) voice counter≦Tvc and Ei is twice the average E and ZCRi>the ZCR threshold. otherwise the state machine will move to transition state 43.

[0041] Referring to FIG. 8A, the VAD state machine implemented in the DSP or core software, begins it initial determination by initialization of a number of parameters 50. The high power level threshold T_(high) is set to a level wherein signals above the level can be classified as voice with great confidence. The lower power level threshold T_(low), is set to a level wherein signals below this level can be classified as noise with a great degree of confidence. A threshold for zero crossing rate is established and all counters are reset. The state machine is initially set to voice state and the frame counter is set to 1.

[0042] The VAD then accepts the current frame of samples 51, divides the frames into sub-frames or blocks (2.5 ms in the exemplary embodiment) and determines block energy Ei and zero crossing rate ZCRi for each sub-frame i and determines the average Ei for each frame. The mean sub-frame Ei for each frame and mean sub-frame ZCRi is also calculated. The state machine then determines the previous state in steps 53 or 54 and process the current frame accordingly as illustrated in FIG. 8B VAD voice state, 8C VAD noise state and 8D VAD transition state. Upon return, 55, the VAD output is set to 1 for voice or transition state and is set to 0 for noise state. The parameters and frame counter are updated 56 and the next frame samples are input 51.

[0043] As illustrated in FIG. 8B, the values for the block energy Ei and the zero crossing rate ZCRi vectors for the current frame are input. The VAD voice state algorithm first determines, if any Ei is above the high threshold Th. If any Ei is greater than T_(high) than the VAD will remain in voice state 41 and flow will return to block 55 in FIG. 8A. If all Ei are less than or equal to T_(low), than the VAD will move to transition state 43 and flow will return to block 55 in FIG. 8A. If Fi is between T_(high) and T_(low), then the algorithm will determine if the voice counter threshold and zero crossing rate threshold have been exceeded. If both have been exceeded, the state will remain voice and flow will return to block 55. If only the zero crossing threshold has been exceeded and the voice counter threshold has not been exceeded, the algorithm compares the current energy Ei to the average energy Eavg and if the current power level is twice the average or greater then the VAD remains in voice state 41, otherwise the VAD moves to transition state

[0044] As illustrated in FIGS. 7 and 8D, once in the transition state 43, the VAD can remain in transition state, move to voice state, or move to noise state. The transition state VAD algorithm, as illustrated in FIG. 8D, receives the same power vectors and first 70 determines if any of the individual Ei values of the sub-frames exceeded Th. If any sub-frame exceeds T_(high) the VAD moves to voice state 41. It is important to move to voice state if any sub-frame exceed T_(high) to avoid valid voice signal being classified and discarded as noise. Alternatively, if the Ei of all sub-frames are below T_(low), 71, then the VAD moves to noise state 45 because the frame is confidently classified as noise if the transition time is longer than its threshold hangover time. The VAD can either remain in transition state or move directly to voice or noise state.

[0045] If the Ei of some of the sub-frames are between the high and low thresholds T_(high) and T_(low), and the means Ei is more than twice the average Ei and more than two ZCRi are above the threshold ZCR 72, the VAD also moves to voice state 41. If the Ei of some of the sub-frames are between the high and low thresholds T_(high) and T_(low), and the transition count is greater than the transition hangover threshold, 73 or the voice count is greater than the voice count short threshold 74 or the threshold for steady noise 75, the VAD moves to noise state 41. If not, the VAD remains in transition state and passes signals as voice.

[0046] As illustrated in FIG. 8C if all Ei is less than T_(high) and the zero crossing rate is less than the zero crossing rate threshold, 61, the VAD noise state algorithm will compare the mean value of Ei to the average value of Ei, 62. If the mean Ei is less than four times the average Ei than the VAD will remain in noise state 43. If the mean Ei is greater than four times the average Ei than the VAD will move to the voice state 41.

[0047] If, at block 61, Ei is not between the two thresholds or the zero crossing rate is greater than the zero crossing rate threshold, the noise count will be compared to the noise count threshold at 63. If the noise count has reached the noise count threshold “noise ct short” 63 and the mean Ei for the frame is greater than the high threshold Th, 65, and the mean Ei is greater than twice the average Ei of the previous E, 66 than the VAD moves to voice state 41. If the relationship between the mean Ei, average Ei and previous E does not move the VAD into voice states the relationship between ZCR and average or mean energy will determine if the VAD moves to voice state.

[0048] Signals produced by voice and noise typically have different characteristics. Voice typically has a greater power level than noise. However, quiet voice can have a power level close to or within the typical range of noise. Voice will however have a greater fluctuation. Therefore the present invention measures not only current power but average and mean power. This can reduce the clipping of the beginning or end of voice which can be at a lower power than sustained voice, It also can reduce the classification of random noise as voice.

[0049] The zero crossing rate is also a feature of the signals which can be used by the present invention. Typically the zero crossing rate for most voice is low and the zero crossing rate for noise is higher. The zero crossing rate for some speech such as consonants is higher than that for noise, while the power level can be lower. These elements of speech can be referred to as un-voice By tracking the zero crossing rate and the distribution, average and mean, a better differentiation between noise and voice can be achieved by the present invention.

[0050] For background noise or white noise the average power remains fairly constant as well as remaining fairly low. For active speech, the average power typically exhibits great variance. Typically voice, active speech, is combined with un-voice, those aspects of speech which exhibit non-typical characteristics or features. This combination produces a feature set of greater variance in characteristics, including power level and zero crossing rate. This difference between speech and background noise is utilized by the present invention to assist in the VAD decisions.

[0051] By distinguishing voice activity from noise activity, certain signals identified as noise can be excluded from further processing. By excluding signals from the comparison process, processing resources can be conserved.

[0052] In order to track a speaker throughout the conference, the speaker is first tracked locally and then the local tracking can be followed across the multiple cores. FIGS. 9A through 9C illustrate the process for single core speaker tracking. First the process is initialized 90 by setting the number of speakers equal to the number of channels in the core. This is the maximum number of speakers possible for a single core. Next the current ith speaker value is set to 0.

[0053] The values for the ith speaker are then obtained from the results of the voice activity detection 91, as described above. The values for: VAD decision, ie. voice or noise; short-term energies Ei; long term energies Em; and variation measures STDi are collected. The validity of the speaker id is then determined based upon the VAD decision. If the signal from the speaker for the current frame was classified as noise by the VAD then the ith speaker is invalid. If the signal from the speaker for the current frame was classified as voice by the VAD then the ith speaker is valid, 92. The speaker number is incremented and values from the VAD are collected for the next speaker until values for all speakers are collected. The speaker tracking algorithm then detects idle channels, 93 and stores the identity of the idle channels for later comparison.

[0054] Next, 94, the number of speakers is compared to the allowed maximum number of active speakers M. The allowed maximum number of active speakers M is the numbers of speakers which are to be transmitted over the conferencing system to the other cores. In the exemplary embodiment described herein, each core can transmit a maximum of three speakers to be compared with the speakers from the other cores to determine the three active speakers for the conference. If the number of valid speakers is three or less, a check of the active speakers is made to determine if the identity of any of the active speakers is on an idle channel 95. Then the active speakers are also evaluated to determine if any of the active speakers are different from the active speakers from the previous frame, 96.

[0055] If there are active valid speakers, the parameters for the active speakers are updated, the features are calculated and the active speaker identifications, parameters and features are communicated to the other cores in the conferencing system 97. The frame is indexed to the next frame, the speaker number is reset 98 and the system gathers the next frame of information for the speakers on the core.

[0056] Otherwise, if there is a change in the identity of the speakers from the previous frame, the speakers are classified according to the signal of the frame under evaluation. Any speakers with a long term energy Em below −42 dBm are identified, as well as the speaker with the minimum long term energy Em(min) and any speaker which was “on” or active in the previous frame.

[0057] The less-than-or-equal-to-M speaker tracking algorithm with speech continuity FIG. 9C, is then implemented. Again the idle channels are removed form the active channels, 100. If any active speakers remain, 101, the speakers are set as the active speakers and the parameters for the active speakers are updated, the features are calculated and the active speaker identifications, parameters and features are communicated to the other cores in the conferencing system 97. The frame is indexed to the next frame, the speaker number is reset 98 and the system gathers the next frame of information for the speakers on the core.

[0058] If no channels remain in step 100, the previous frame of active speakers is checked and the idle channels are removed 102. If the vad id is non-zero, the channels are continued and the speaker id is taken from the vad id, 104 by choosing the remaining active speaker list channel speaker assignment otherwise continue with the previous frame assignment. The parameters for the active speakers are then updated, the features are calculated and the active speaker identifications, parameters and features are communicated to the other cores in the conferencing system 97. The frame is indexed to the next frame, the speaker number is reset 98 and the system gathers the next frame of information for the speakers on the core. If there are no remaining vad id's then in step 103 the speaker assignment from the previous frame is continued.

[0059] If the number of speakers in step 94 is greater than the allowed maximum number of active speakers M not all speakers can be sent to the conference and the speakers to be included in the conference must be identified. However, if the number valid speakers 105 is less than the allowed maximum number of active speakers M but greater than zero, all valid speakers are passed to the M-or-less-than-M speaker tracking algorithm, FIG. 9C. If the number valid speakers 105 is greater than the allowed maximum number of active speakers M the more-than-M speaker tracking algorithm, FIG. 9B, is implemented.

[0060] If there are no valid speakers, 106, then the active speaker from the previous frame, if one exists, 107, is set as the new active speaker, 108, unless this speaker is on an idle channel, 109, then the new speaker is the previous speaker with the smallest Ei, 110. If there are no previous active speakers, 107, the new active speakers are the current speakers with the largest Ei, 111.

[0061]FIG. 9B illustrates the algorithm for determining which speakers to track when the number of speakers and the number of valid ids both exceed the allowable maximum number of active speakers. When there are more valid and active speakers than allowed (three speakers are allowed in the exemplary embodiment), then the M number of strongest speakers must be identified to be passed through to the conference.

[0062] If there are no previous active speakers, 112, the active voice (vad) speakers with the largest Ei are identified, 113, and processing returns to block 97 of FIG. 9A. The parameters for the active speakers are then updated, the features are calculated and the active speaker identifications, parameters and features are communicated to the other cores in the conferencing system 97. The frame is indexed to the next frame, the speaker number is reset 98 and the system gathers the next frame of information for the speakers on the core until the last frame is

[0063] If some of the speakers which were previously active speakers remain as active valid speakers in step 112, then, the idle channels are removed, 114 and the remaining # of active speakers is compared to the maximum allowable number, 115. In the exemplary embodiment, if the number is three or less, the speakers are processed according to the three-or-less speaker tracking algorithm of FIG. 9C. If the number of active, valid, non-idle-channel speakers remains greater than three, the strongest three or less speakers with engstd>threshold and Th_Energy_Std_low are determined by a comparison of the signal features of each speaker as illustrated in FIG. 9D.

[0064]FIG. 9D illustrates the implementation of the speaker selection based upon signal features. The id's of the speakers identified as active voice by the VAD; the current active speakers id; the energy and standard energy; noise and number of speakers are input 116. The first time through this algorithm, the threshold energy standard deviation is set high. If theses settings result in no remaining active speakers, 117, then flow returns to FIG. 9B, block 118, otherwise the number of speakers with energy above the noise threshold is determined, 119. If the number of speakers with energy above the noise standard deviation threshold is less than the maximum number of speakers allowed 120 then these speakers are set as the new active speakers 121. The new active speaker ids are removed from the vad id and the speaker on id, 127 and flow returns to FIG. 9, block 118.

[0065] If the number of speakers in 120 is above the maximum number M, then the number of new speakers in the group is determined 122. If there are more new speakers than the allowable maximum M, 123, than the speakers with the largest energies are selected 124. The new active speaker ids are removed from the vad id and the speaker on id, 127 and flow returns to FIG. 9B, block 118. If there are an insufficient number of new speakers at 123, the new speakers are removed from the number of high speakers 125 and the number of speakers with the largest energies are selected, 126.

[0066] Upon return to block 118, if the number of active speakers is now less than the maximum allowable number of speakers, than the speaker selection algorithm FIG. 9D is again run with the threshold standard energy set low to add more speakers. If upon return to block 128, if the number of active speakers remains lower than the maximum allowable number of speakers, than the speaker selection algorithm FIG. 9D is again run with the threshold standard energy set at its limit to add additional speakers until the number of active speakers is determined.

[0067] Because many varying and different embodiments may be made within the scope of the inventive concept herein taught, and because many modifications may be made in the embodiments herein detailed in accordance with the descriptive requirements of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense. 

What is claimed is:
 1. A single core speaker tracking system in a distributed conferencing system, comprising: a system communication bus; a plurality of nodes connected to said system communication bus, each of said plurality of nodes comprising: a state machine for receiving a signal from each of a plurality of local participants connected to said node for classification of said signals as voice or noise and for producing a indicator of the classification of each of said signals; feature extractors connected to said state machine for receiving said indicator and to said local participant signals for measuring and extracting at least one feature of each of said local participant signals classified as voice; a speaker tracker connected to said feature extractors for selecting a number of said local participant signals for tracking based upon said extracted features; a multiplexer connected to said speaker tracker, said local participant signals and said system communication bus, for passing said selected local participant signals to said system communication bus and for blocking the remaining local participant signals from said system communication bus.
 2. A single core speaker tracking system of claim 1, wherein: said multiplexer communicates said features of each of said selected signals to said plurality of conferencing nodes.
 3. A single core speaker tracking system according to claim 1, wherein: said speaker tracker uses a high threshold to select signals and a low threshold to exclude signals from selection.
 4. A single core speaker tracking system according to claim 1, wherein: said speaker tracker selects said signals on the basis of the measured energy level of said signal.
 5. A single core speaker tracking system according to claim 1 wherein: said speaker tracker considers the previous selected status of each speaker in determination of the current selection of speakers.
 6. The single core speaker tracking system of claim 1 wherein said selection of local participant signals is based upon extracted parameters of said signals to select the strongest speaker signal while maintaining channel continuity of previously selected speaker signals.
 7. A method for tracking speakers on a single core of a multi-core distributed conferencing system, comprising the steps of: setting a maximum number of active speakers to be transmitted by each core; determining the number of channels on said single core; determining the voice or noise state of each speaker on said single core; assigning a valid identifier to each speaker signal on said single core which is identified as a voice signal; extracting values for the short-term energies, long term energies, and energy variation of each speaker signal identified as voice; detecting idle channels; comparing the number of assigned valid speakers to the allowed maximum number of active speakers and designating the valid speakers as current active speakers when the number of valid speakers does not exceed the allowed maximum number of active speakers; updating the parameters and features for the current active speakers based upon the features extracted from the valid speakers when the identity of the current active speakers is the same as the identity of the previous active speakers; communicating the current active speaker identifications, parameters and features to the other cores in the conferencing system when said active speakers are not on an idle channel.
 8. The method for tracking speakers of claim 7, further comprising the steps of: ordering the speakers according to the long term and short term energies of the current signals when there is a change in the identity of one or more of the speakers from the previous frame; sequentially selecting the valid speakers beginning with the speaker having the highest energy as the current active speakers.
 9. The method for tracking speakers of claim 7, further comprising the steps of: reclassifying the previous active speakers as the current active speakers when the number of current valid speakers is less than one.
 10. The method for tracking speakers of claim 7, further comprising the steps of: ordering the speakers according to the long term and short term energies of the current signals when there are more valid speakers than the allowed maximum number of active speakers; sequentially selecting the valid speakers beginning with the speaker having the highest energy as the current active speakers.
 11. A method for tracking speakers on a single core of a multi-core distributed conferencing system, comprising the steps of: setting a maximum number of active speakers to be transmitted by each core; determining the number of channels on said single core; determining the voice or noise state of each speaker on said single core; assigning a valid identifier to each speaker signal on said single core which is identified as a voice signal; extracting values for the short-term energies, long term energies, standard energy and energy variation of each speaker signal identified as voice; detecting idle channels; comparing the assigned valid speakers to the allowed maximum number of active speakers and comparing the energy of each valid speaker to a threshold energy standard deviation when the number of valid speakers exceeds the allowed maximum number of active speakers; determining the number of speakers with energy above the noise threshold; updating the parameters and features for the current active speakers based upon the features extracted from the valid speakers above the threshold, when number of valid speakers above the threshold is equal to the allowed maximum number of active speakers; ordering the speakers according to the long term and short term energies of the current signals when there are more valid speakers above the threshold than the allowed maximum number of active speakers; sequentially selecting the valid speakers beginning with the speaker having the highest energy as the current active speakers; lowering the threshold energy standard deviation when the number of valid speakers above the previous threshold energy standard deviation is less than the allowed maximum number of active speakers and re-comparing the assigned valid speakers to the revised lowered threshold; communicating the current active speaker identifications, parameters and features to the other cores in the conferencing system when said active speakers are not on an idle channel. 